In monochrome display tubes the electron gun system is designed to generate one electron beam, whereas in colour display tubes of the in-line type the electron gun system is designed to generate three coplanar electron beams which convergence on the display screen.
The deflection unit for deflecting the electron beams which is placed around the display tube is used to deflect the electron beams from their normal undeflected straight path in one or in the other direction so that the beams impinge upon selected points of the display screen so as to provide visual indications thereon. By varying the magnetic deflection fields in a suitable manner the electron beams can be moved upwards or downwards and to the left or to the right over the (vertically disposed) display screen. By simultaneously varying the intensity of the beams a visual presentation of information or a picture can be formed on the display screen. The deflection unit connected around the neck portion of the display tube comprises two deflection coil systems so as to be able to deflect the electron beams in two directions which are transverse to each other. Each system comprises two coils which are placed on oppositely located sides of the tube neck, the systems being moved relative to each other 90.degree. around the tube neck. When the two deflection coil systems are energized they produce orthogonal deflection fields.
Essentially the fields are at right angles to the path of the undeflected electron beams. A cylindrical core of magnetisable material which may tightly fit around the deflection coil systems when the deflection coil systems are both of the saddle type, is usually used to concentrate the deflection fields and to increase the flux density in the deflection area.
In order to meet certain requirements as regards the picture quality, the (dynamic) magnetic deflection fields should often be modulated strongly. For example, the more and more stringent requirements as regards the convergence in three-in-line colour television systems necessitate, in addition to a strong negative magnetic sixpole component in the central area of the frame deflection field, a strong positive magnetic sixpole component on the gun side of the frame deflection field. The strong positive sixpole component is necessary for coma correction. (The effect of a positive sixpole component on the dipole deflection field is a pincushion-shaped field variation). For a self-convergent in-line colour system having green as the central beam and red and blue as the outer beams, coma is to be understood to mean: a vertical shift from red and blue with respect to green. When no coma correction measures are taken, red and blue will be deflected more strongly than green. In a pincushion-shaped deflection field on the gun side, red and blue experience a weaker deflection field than green. As a result of this, red and blue will be deflected less strongly.
Display devices of the kind discussed above comprise frame deflection coils of the saddle type. These are self-supporting coils which comprise a number of conductors which are wound so as to produce first and second side packets, an arc-shaped first end section and an arc-shaped second end section which together define a window. In such coils the rear end sections (on the gun side) may be directed upwards with respect to the profile of the display tube (the original type of saddle coil) or be directed downwards (in this type of saddle coil the rear end section, as it were, follows the tube profile).
A pincushion-shaped field is generated when the window apertures of the two saddle coils of a deflection coil system are large, a barrel-shaped field is generated when the window apertures are small. For a self-converging system the frame deflection field in the central area must have a barrel-shaped distribution (the individual saddle frame deflection coils must thus have a small window aperture), on the gun side it must have a pincushion-shaped distribution (large window aperture) and on the cup side it must have a homogeneous or more or less pincushion-shaped distribution dependent on the fact how much east-west frame distortion is to be permitted. Such field distributions are also of importance for monochrome display tubes/deflection unit systems which are to have a high resolving power.
It has so far not proved possible to make saddle coils having a window aperture which varies as strongly as is desired for the said applications while using the currently used winding methods. On the other hand, several compromise solutions are known to make the problem less great. For example, a less large window than is in fact necessary will suffice on the gun side by intensifying the pincushion shape of the frame deflection field at the area by means of segments of soft-magnetic metallic material placed in the frame deflection field. However, from an energetic point of view the use of such segments is undesired.